Chart recorders are used in various applications where it is necessary or desirable to make a permanent visual record of variable data. Although chart recorders of various types and sizes are known to the art, the typical chart recorder moves an elongated web or strip of recording medium relative to a device which marks the recording medium in response to the variable data being recorded. Various kinds of recording media are known, including plain paper marked with a pen stylus and various coated recording papers marked by a thermal or electric spark stylus.
The recording paper typically is supplied in roll form to the chart recorder, which feeds paper from the roll at a selected linear speed past the marking stylus. The marked paper may then move past a display window or region for immediate viewing, and may thereafter wind onto a suitable takeup spindle for storage. Alternatively, some chart recorders, such as EKG and other medical data recorders, dispense the marked paper for tearoff and immediate separation from the chart recorder.
Factors such as size limitations and paper speed limit the maximum amount of recording time available in a chart recorder without adding a fresh supply of recording paper. Consequently, a fresh supply of recording paper often must be added when it is least convenient to do so, namely, while other activities are taking place. For example, recording marine depth sounders are often used by fishermen to locate fish or promising fishing locations. Changing recording paper in a conventional chart recorder requires opening a door or face plate to the recorder, removing the roll of used paper, removing the core remaining from the exhausted paper roll, inserting a fresh roll of paper, and threading this paper through a convoluted path past the marking stylus and one or more paper guides for attachment to the paper winding spindle. Accomplishing all these steps with a conventional marine chart recorder in a small fishing boat is tedious, time-consuming and frustrating, especially where the boat may be moving and the person's hands may be slippery from handling bait or fish.
A number of other problems exist with recording marine depth sounders and other chart recorders of the prior art. For example, the recording paper transport mechanism found in such recorders includes exposed gears or other drive train elements. These gears are subject to becoming coated with dust or dirt, such as rubber dust from drive belts or powdery material from the coating used on the chart paper. The efficiency and longevity of such exposed drive trains is thus impaired.
The manner of moving chart paper through chart recorders has also presented problems, especially where relatively slow linear speeds are desired. The recording paper must move past the marking stylus at a substantially constant selected speed, inasmuch as paper speed fluctuations produce a nonlinear time base against which depth or other measured data is recorded. Many chart recorders have sought to provide linear paper drive by means of a power-driven pinch roller or capstan which engages and pulls the chart paper past the printing stylus. A separate drive connection to the paper takeup shaft is also required, capable of driving the takeup shaft at variable speeds as the effective diameter increases due to paper winding. Takeup shaft drive in the prior art recorders is usually accomplished through a slip clutch, which further complicates the paper drive arrangement. Such paper takeup slip clutches sometimes fail to provide the necessary "slip" or overrun, particularly at relatiely low linear paper speeds, causing the takeup shaft to pull the chart paper faster than the nominal paper speed of the pinch rollers. This problem is frequently enhanced by a lack of paper-pulling torque sometimes exhibited at low linear paper speeds, due to slippage or other inefficiencies of the pinch roller arrangement.
Other problems associated with chart recorders of the prior art are addressed with reference to the improvements set forth in the following disclosed embodiment of the present invention.